The present invention relates to a method for traction control and a traction control system.
When starting off or accelerating a vehicle on a slippery road surface, a traction control system (TCS) automatically intervenes in the operation of the vehicle if at least one drive wheel begins to slip and exceeds a specified slip threshold. To this end, the engine torque is as a rule reduced by adjusting the throttle valve. If necessary, the slipping drive wheels may also be decelerated by a braking intervention. After having dropped below the specified slip threshold, the engine torque is then increased again.
FIG. 1 shows the curve of an engine torque 1 during a TCS regulation. The setpoint engine torque specified by the regulation is denoted by reference notation 1, the actual torque by reference notation 2.
If speed 10 (see FIG. 2) exceeds a specified slip threshold at point in time t0, the TCS abruptly reduces engine torque 1 to a pre-control value 5. The degree of setback 4 is ordinarily a function of the sum of the running resistances.
Setpoint engine torque 1 or pre-control value 5 is subsequently increased during a rate time 6 as a function of vehicle acceleration 12 (see FIG. 2). As can be seen, the vehicle starts to move off and accelerate from point in time t1. Pre-control value 5 also increases correspondingly.
During rate time 6, the TCS control loop is still not closed, so that control of setpoint engine torque 1 actually occurs in this phase.
A high wheel slip occurs not only when starting off on a slippery road surface but also when starting off on a non-skid road surface if the driver engages the clutch hard while applying high torque. Since the vehicle attains only a fraction of the ultimate acceleration during rate time 6 and it is only possible to make an extremely rough estimate of the adhesive friction value of the road surface, in this case a very low pre-control value 5 is selectedxe2x80x94as if the vehicle were on a slippery road surfacexe2x80x94although the road surface actually has a very high coefficient of friction.
The consequence of this TCS regulation is the curve of vehicle acceleration 12 shown in FIG. 2 with an acceleration drop 13. The degree of acceleration drop 13 (see reference notation 18) may assume values exceeding 1 m/s2. The driver, who would actually like to accelerate rapidly, perceives a drop in acceleration by more than 1 m/s2 as very unsatisfactory.
After rate time 6, the setpoint engine torque is reduced still further since the slipping drive wheel is again being regulated. Only at point in time t3 does speed 10 of the driven wheel again drop back below the specified slip threshold, as a result of which setpoint engine torque 1 is subsequently increased again (see control phase 8).
According to an embodiment of the present invention vehicle acceleration is improved by prolonging the rate time of a traction control system in a starting-off operation, if the vehicle acceleration or speed exceeds a specified threshold value, and by increasing the setpoint engine torque during the prolonged rate time as a function of the vehicle acceleration or speed. If the vehicle attains high acceleration values or speeds within the original rate time (usually 180 ms), this is an indication of a non-skid road surface having a high coefficient of friction. The prolongation of the rate time may cause the vehicle to be accelerated further before the start of the actual TCS regulation, which reduces the engine torque. In one embodiment the rate time is prolonged to at least 250 ms. In another embodiment, the rate time is prolonged to at least 300 ms or more.
The prolongation of the rate time preferably occurs only if the engine torque or drive torque exceeds a specified threshold value. The prolongation of the rate time also preferably occurs only when the vehicle is started off or at low speeds, e.g., less than 10 km/h.
Usually, an attenuation factor is taken into consideration when calculating the pre-control value or the setpoint engine torque. According to one embodiment of the present invention, this attenuation factor is changed as a function of the vehicle acceleration; in particular, it is increased with increasing vehicle acceleration. The attenuation factor may also assume values greater than one.